The present invention is primarily concerned with detection of the relative positions of a piston and cylinder of a hydraulic or similar cylinder so that a value can be provided to a control system using the cylinder of the current location of the piston in the cylinder and thus the current location of an object controlled by that cylinder. However many of the principles disclosed herein can be used in detection of the distance between any two elements and the present application is therefore not limited to hydraulic cylinders.
The provision of a value representing displacement of a cylinder can be used in a system controlling the cylinder for many different end uses and purposes. One such purpose is for safety to ensure that the cylinder and theus the object is not moved to a specific location in dangerous circumstances such as when a door is open. Another such purpose is to provide interaction between different functions so that another element is controlled in dependence upon the specific location of a cylinder and thus the object, such as to maintain a bucket level as an arm is raised, for which function the location of the arm must be known.
The current technology used to measure hydraulic cylinder stroke position requires a hole be drilled the length of the cylinder shaft or the piston rod (gun drilling) and the sensor installed within. Repair, replacement or retrofit with this type of sensor is not practical as a serviced part. This arrangement is expensive and ineffective so that it has not yet met significant commercial success.
Some attempts have been made to use techniques using reflected sound waves and resonant frequencies but this technique is temperature dependent thus requiring temperature detection and compensation calculations so that the technique has not lead to any success.
In Published US Application 2002-0064300 of the present Assignees published May 30, 2002 there is described a method for detecting relative displacement between an object such as a slow moving vehicle and an illuminated surface which is normally the ground surface illuminated by an infrared light source so as to provide an output indicating the displacement or a velocity calculated from the displacement. The method includes providing an array of CCD or similar elements each arranged to receive light from a portion of a field of view and to provide an output responsive thereto. The method involves selection of a reference image and repeatedly comparing the reference image of the surface with each successive image by calculating the convolution integral of the signals using a fast Fourier transform to obtain a probable displacement value. The reference image is maintained as long as possible until a “Q” factor falls below an acceptable minimum, or until a predetermined time elapses or until a predetermined displacement is measured. The displacement values obtained from the comparisons can be filtered by discarding some values if they are outside an expected range of probable values.